·noun Any one of several species of large oceanic fishes belonging to the Mackerel family, especially the common or great tunny (Orcynus / Albacora thynnus) native of the Mediterranean Sea and the Atlantic Ocean. It sometimes weighs a thousand pounds or more, and is extensively caught in the Mediterranean. On the American coast it is called horse mackerel. ·see ·Illust. of Horse mackerel, under Horse.